1911 Encyclopædia Britannica/Greensand
GREENSAND, in geology, the name that has been applied to no fewer than three distinct members of the Cretaceous System, viz. the Upper Greensand (see Gault), the Lower Greensand and the so-called Cambridge Greensand, a local phase of the base of the Chalk (q.v.). The term was introduced by the early English geologists for certain sandy rocks which frequently exhibited a greenish colour on account of the presence of minute grains of the green mineral glauconite. Until the fossils of these rocks came to be carefully studied there was much confusion between what is now known as the Upper Greensand (Selbornian) and the Lower Greensand. Here we shall confine our attention to the latter.
The Lower Greensand was first examined in detail by W. H. Fitton (Q.J.G.S. iii., 1847), who, in 1845, had proposed the name “Vectine” for the formation. The name was revived under the form “Vectian” in 1885 by A. J. Jukes-Browne, because, although sands and sandstones prevail, the green colour has often changed by oxidation of the iron to various shades of red and brown, and other lithological types, clays and limestones represent this horizon in certain areas. The Lower Greensand is typically developed in the Wealden district, in the Isle of Wight, in Dorsetshire about Swanage, and it appears again beneath the northern outcrop of the Chalk in Berkshire, Oxfordshire and Bedfordshire, and thence it is traceable through Norfolk and Lincolnshire into east Yorkshire. It rests conformably upon the Wealden formation in the south of England, but it is clearly separable from the beds beneath by the occurrence of marine fossils, and by the fact that there is a marked overlap of the Lower Greensand on the Weald in Wiltshire, and derived pebbles are found in the basal beds. The whole series is 800 ft. thick at Atherfield in the Isle of Wight, but it thins rapidly westward. It is usually clearly marked off from the overlying Gault.
In the Wealden area the Lower Greensand has been subdivided as follows, although the several members are not everywhere recognizable:—
|Isle of Wight.|
|Folkestone Beds (70-100 ft.)||Carstone and Sand rock series.|
|Sandgate Beds (75-100 ft.)||Ferruginous Sands (Shanklin sands).|
|Hythe Beds (80-300 ft.)||Ferruginous Sands (Walpen sands).|
|Atherfield Clay (20-90 ft.)||Atherfield Clay.|
The Atherfield Clay is usually a sandy clay, fossiliferous. The basal portion, 5-6 ft., is known as the “Perna bed” from the abundance of Perna Mulleti; other fossils are Hoplites Deshayesii, Exogyra sinuata, Ancyloceras Malhesonianum. The Hythe beds are interstratified thin limestones and sandstones; the former are bluish-grey in colour, compact and hard, with a certain amount of quartz and glauconite. The limestone is known locally as “rag”; the Kentish Rag has been largely employed as a building stone and roadstone; it frequently contains layers of chert (known as Sevenoaks stone near that town). The sandy portions are very variable; the stone is often clayey and calcareous and rarely hard enough to make a good building stone; locally it is called “hassock” (or Calkstone). The two stones are well exposed in the Iguanodon Quarry near Maidstone (so called from the discovery of the bones of that reptile). Southwest of Dorking sandstone and grit become more prevalent, and it is known there as “Bargate stone,” much used around Godalming. Pulborough stone is another local sandstone of the Hythe beds. Fuller's earth occurs in parts of this formation in Surrey. The Sandgate beds, mainly dark, argillaceous sand and clay, are well developed in east Kent, and about Midhurst, Pulborough and Petworth. At Nutfield the celebrated fuller's earth deposits occur on this horizon; it is also found near Maidstone, at Bletchingley and Red Hill. The Folkestone beds are light-coloured, rather coarse sands, enclosing layers of siliceous limestone (Folkestone stone) and chert; a phosphatic bed is found near the top. These beds are well seen in the cliffs at Folkestone and near Reigate. At Ightham there is a fine, hard, white sandstone along with a green, quartzitic variety (Ightham stone). In Sussex the limestone and chert are usually lacking, but a ferruginous grit, “carstone,” occurs in lenticular masses and layers, which is used for road metal at Pulborough, Fittleworth, &c.
The Lower Greensand usually forms picturesque, healthy country, as about Leith Hill, Hindhead, Midhurst, Petworth, at Woburn, or at Shanklin and Sandown in the Isle of Wight. Outside the southern area the Lower Greensand is represented by the Faringdon sponge-bearing beds in Berkshire, the Sandy and Potton beds in Bedfordshire, the Shotover iron sands of Oxfordshire, the sands and fuller's earth of Woburn, the Leighton Buzzard sands, the brick clays of Snettisham, and perhaps the Sandringham sands of Norfolk, and the carstone of that county and Lincolnshire. The upper ironstone, limestone and clay of the Lincolnshire Tealby beds appear to belong to this horizon along with the upper part of the Speeton beds of Yorkshire. The sands of the Lower Greensand are largely employed for the manufacture of glass, for which purpose they are dug at Aylesford, Godstone, near Reigate, Hartshill, near Aylesbury and other places; the ferruginous sand is worked as an iron ore at Seend.
This formation is continuous across the channel into France, where it is well developed in Boulonnais. According to the continental classification the Atherfield Clay is equivalent to the Urgonian or Barremian; the Sandgate and Hythe beds belong to the Aptian (q.v.); while the upper part of the Folkestone beds would fall within the lower Albian (q.v.).
(1875), “Geology of the Isle of Wight” (2nd ed., 1889), “Geology of the Isle of Purbeck” (1898); and the Record of Excursions,Geologists' Association (London, 1891).
(J. A. H.)